The term “miniaturized sensor” is used to mean a sensor that presents at least one dimension lying in the range a few micrometers to a few hundreds of micrometers.
Such sensors may be used for measuring a flow speed of a fluid, wall shear friction, or indeed a pressure.
A miniaturized heater element sensor is described for example in the article by Meunier et al., “Realization and simulation of wall shear stress integrated sensors”, Microelectronics Journal 34 (2003), pp. 1129-1136, for measuring the wall shear stress associated with a fluid flow.
That sensor comprises a substrate, a thermally insulating structure, a heater element (hot wire), and electrical contacts. The thermally insulating structure makes it possible to define a cavity over which the hot wire extends, the hot wire being held on the thermally insulating structure via its ends. The electrical contacts are arranged on the thermally insulating structure and they are connected to the ends of the hot wire, so that the hot wire can be electrically powered to perform heating by the Joule effect.
Several parameters are involved in the quality of the measurement taken with a sensor of that type.
Firstly, it is necessary to consider the shape of the hot wire.
It is preferable for the hot wire to present the greatest possible length Lwire with a hydraulic diameter dh that is as small as possible. The hydraulic diameter dh is defined by the relationship:dh=4S/P where S is the section of the hot wire and P is its wetted perimeter.
A small hydraulic diameter for the hot wire limits the thermal inertia of the wire and thus improves the bandwidth of the sensor.
Furthermore, in practice, the person skilled in the art considers that it is preferable to dimension the wire so that Lwire/dh>30, in order to obtain a sensor that is sufficiently sensitive. This relationship is satisfied by the sensor proposed by Meunier et al.
The shape of the hot wire thus has an influence on the sensitivity of the sensor and on its bandwidth.
It is then also necessary to consider the nature of the materials used for making the wire.
The natures of the materials that are selected has an effect on the temperature coefficient of resistance (TCR) of the wire and consequently on the sensitivity of the sensor.
Furthermore, the natures of the materials selected for the wire define the thermal conductivity of the wire, which needs to be as high as possible in order to improve the bandwidth of the sensor.
In the article by Meunier et al., a hot wire is selected for this purpose that is made of boron-doped polysilicon.
Finally, in order to avoid interfering heat losses, which also have an influence on the bandwidth of the sensor, it is appropriate to insulate the hot wire thermally from the substrate as much as possible.
For this purpose, the hot wire of the sensor proposed by Meunier et al. extends over a cavity containing air, which by its nature is a poor conductor of heat.
Also for this purpose, a thermally insulating structure is used between the ends of the hot wire and the substrate. Specifically, the thermally insulating structure is made of silicon nitride since, like air, that material presents low thermal conductivity.
The sensor proposed by Meunier et al. is relatively simple in design and presents sensitivity that is sufficient for measuring wall shear friction.
Nevertheless, that type of sensor can be difficult to use for measurements other than those concerning measuring wall shear friction or measurements of low flow speed.
The hot wire is fragile and it is difficult to use for measuring speeds typically exceeding 20 meters per second (m/s), since otherwise it risks breaking. This fragility is associated with the fact that the hydraulic diameter dh of the wire is small, its ratio Lwire/dh is high, and it is suspended over the cavity.
Numerous heater element sensors of different designs have been proposed.
The sensor proposed by Chiang Lu et al., “A micromachined flow shear-stress sensor based on thermal transfer principles”, Journal of Microelectromechanical Systems, Vol. 8 (1), pp. 90 to 99, is particularly advantageous.
That sensor comprises a substrate having a cavity, a thermally insulating structure suspended over the cavity by means of connection zones connecting it with the substrate, and a heater element (hot wire) arranged on the thermally insulating structure.
In that design, the thermally insulating structure is in the form of a diaphragm suspended over a cavity made in the substrate.
The hot wire satisfies the relationship Lwire/dh>30, and it is made of phosphorus-doped polysilicon. The characteristics of the wire (shape, TCR, thermal conductivity, . . . ) are thus close to the characteristics of the hot wire used in the sensor proposed by Meunier et al.
Furthermore, the thermally insulating structure used by Chiang Lu et al. presents thermal conductivity identical to the thermally insulating structure of the Meunier et al. sensor, insofar as it is likewise made of silicon nitride.
The sensitivity of that sensor should be comparable to that of the sensor proposed by Meunier et al.
Nevertheless, that sensor differs from the sensor proposed by Meunier et al. mainly in the shape of the thermally insulating structure (diaphragm), which enables the cavity made in the substrate to be covered so that it can contain a high vacuum.
Insofar as the hot wire is supported by the diaphragm, it can be understood that the sensor is more robust than the sensor proposed by Meunier et al. The robustness of that sensor is also improved by the greater thickness of the hot wire (0.45 micrometers (μm) for Chiang Lu et al. as compared with 0.3 μm for Meunier et al.).
It is therefore possible to envisage using the sensor structure proposed by Chiang Lu et al. to perform measurements of flow speeds that are higher than can be measured with the sensor proposed by Meunier et al. The article by Chiang Lu et al. also specifies that their sensor has been tested at speeds of 25 m/s.
In contrast, that sensor gives a bandwidth comparable to that of the bandwidth of the sensor proposed by Meunier et al., in spite of using a cavity under a high vacuum under the thermally insulating diaphragm, the cavity limiting exchanges of heat between the diaphragm and the cavity.
The authors specify that the cutoff frequency of their sensor is 1.9 kilohertz (kHz) (at constant current), whereas the cutoff frequency of the sensor proposed by Meunier et al. is about 2 kHz (at constant current).
This is probably explained by the heat losses associated with conduction in the diaphragm and by the greater thickness of the hot wire. Using a high vacuum under the diaphragm thus appears to do more than compensate for the drawbacks in terms of bandwidth that are associated with using a thermally insulating diaphragm and a greater thickness for the hot wire.